One Punch is one of the many modern Shounen that take its roots from DBZ.... So I would have to say that Saitama would likely win. As for superman.... Gets knocked out by missiles and robots, hits mortal DC villains without restraint but miraculously doesn't kill them.... Somehow can tow planets, survive a supernova, and breathe in space. I base my argument on classic superman before he was OP as hell and limit goku to pre-super saiyan max (meaning training at 100 times gravity). Screw attack didn't account for character and canon consistency in their battle or the fact that ki blast are plasma. When you take away the inconsistency and get at the core characters it works out like this.
Saitama beats Goku and superman finishes last.
The simple reason this will never truly resolve is because DC writes around any of superman's weaknesses and makes him stronger each time a character comes along that threatens his supremacy. Last time he was majorly rebooted (before recent DC overhaul) was in the 1980's-90's. Gee I wonder what character could have been rising to power at that time that made them transform superman from hero to god...... Oh wait.... That was when DBZ was airing lol